This invention relates to trigger circuits; and more particularly, to a trigger circuit designed for providing a current pulse to a 55-Nitinol wire to heat the wire to the temperature required to restore the wire to its original length from a stretched condition.
An alloy of nickel and titanium of particular stoichiometer composition that exhibits a dimensional memory has been developed. A length of wire of this alloy has the following characteristics: a given length of wire can be stretched up to (about 10 percent of its original length) but not exceeding a predetermined maximum for the alloy and when the tensile stress used to stretch the wire is removed, the wire remains at this stretched length indefinitely provided that its temperature is not elevated above its martensitic critical temperature. When the temperature of the stretched wire is elevated to the martensitic critical temperature of the alloy, the wire is substantially instantly restored to its original dimension and during restoration, the wire exhibits a substantial tensile force. In fact, when the wire restores to its original length, it exhibits a contracting force several times greater than the tensile force required to stretch the wire at its lower temperature.
If one end of a stretched Nitinol wire is fixed and the other end is free, the free end will move toward the fixed end when it is heated to its critical temperature. Thus, if a mechanism such as a photographic shutter, for example, is operatively secured to the free end of the wire, the mechanism can be operated by the force exhibited by the Nitinol wire when it is restored to its unstretched state.
A more detailed description of Nitinol wire and the use of such wire for activating photographic shutters is given in U.S. Pat. No. 3,883,885 and the entire disclosure of this U.S. Pat. No. 3,883,885 is incorporated herein by reference.